1. Field of the Invention
The present invention relates to an optical recording medium, and more particularly, to an optical recording medium of a single plate type, capable of recording a large volume of information and playable by commercially available compact disc players, which comprises a recording layer containing a dye, a reflective layer overlying the recording layer, and a protective layer overlying the reflective layer.
2. Description of the Related Art
As an information-oriented society has recently developed rapidly, optical recording mediums have been widely utilized and researched which can make a remarkably high density recording as compared with magnetic recording mediums.
As optical recording mediums, there are known a read-only-medium in which information has been recorded in advance and only reproduction is possible, a writable medium in which information can be recorded by users and reproduced, and a rewritable medium in which information can be recorded therein and the information thus recorded can be reproduced or erased.
Among them, the writable medium is generally inexpensive and excellent in durability as compared with the rewritable medium. Therefore, the writable medium has been widely used as a medium for storing a large volume of data.
The optically writable medium may have a metal layer of Te, Bi or the like or a dye layer of cyanine dyes, phthalocyanine dyes or the like as a recording layer on a substrate.
Information can be recorded in the recording layer by irradiating a laser beam to cause a physical or chemical change (pits formation) in the layer.
The pits are read out with a laser beam which is sufficiently weaker than that used for recording and thereby the information can be reproduced.
Among the optically writable mediums, there have recently been developed and spread some mediums capable of reading out by commercially available compact disc (CD) players.
The writable mediums which can be reproduced by CD players are proposed in Optical Data Storage 1989 Technical Digest Series Vol. 1, 45 (1989), EP-353393 and the like. The mediums are of a single plate type comprising a recording layer composed of an organic dye, a metal reflective layer and a protective layer. However, because a cyanine dye is used in the recording layer, there are problems as to poor light resistance and poor heat and humidity resistance.
Further, EP-0373643 discloses a writable medium reproducible by CD players and having a recording layer composed of a phthalocyanine dye. When a phthalocyanine dye, in particular, a phthalocyanine dye soluble in a nonpolar solvent is used in a recording layer, the resulting medium is excellent in light resistance and heat and humidity resistance, but there are some drawbacks that under certain conditions good written (recording) signal characteristics are not obtained, when a great number of sheets of each individual medium has a very small difference in recording signal characteristics; and the recording sensitivity of the medium is low.
For forming a recording layer containing a dye of the above-mentioned medium, there is usually used a method for coating with an organic dye solution. In particular, a spin-coating method is generally used from the standpoints of easy film formation and economy.
However, there have been hardly any investigation of the amount of a solvent remaining in a recording layer and the drying conditions after forming the recording layer when the recording layer is formed by a coating method.
Japanese Patent Application Laid-open No. Hei 2-16083 discloses that a solvated dye is used for a recording layer. And a dye solvated with 0.01-5% by weight of a polar solvent is preferable from the standpoints of jitter and error rate. However, the medium is not provided with a reflective layer and a protective layer formed on a dye recording film, and the medium concerns a cyanine type dye and a polar solvent. This Japanese Patent Application does not disclose a medium comprising a recording layer composed of a nonpolar solvent-soluble phthalocyanine dye, and a reflective layer and a protective layer formed successively on the recording layer.
The present inventors have intensively conducted research to solve the above-mentioned problems of a writable medium capable of read out by a commercial CD player which comprises a recording layer containing a nonpolar solvent-soluble phthalocyanine dye, and a reflective layer and a protective layer successively formed on the recording layer. As a result, it has been found that the amount of a solvent remaining in the recording layer of the medium has a considerable effect on the above-mentioned problems, and the drying conditions after forming the recording layer is very important. Thus the present invention has been completed.